Nationalism can be looked at a couple of different ways, though when most people use the term they're talking about one of three things.
- Nationalism can mean people living in a nation coming to see themselves, primarily, as members of that nation rather than distinct separate groups. These groups may be ethnic, religious, or ideologically based. The idea behind nationalism is that people will put these differences second and accept the common traits of the nation first.
- Nationalism can also mean a strong belief in the superiority of one particular nation over others. This is love of nation over other nations, even to the point where wars are started.
- Nationalism can be used as a rallying cry by majority groups who want to exclude minority groups because they don't "fit in" with the national identity (that of the majority group.)
When it comes to the American Revolution, the first definition is probably most apt. The colonists were in a tight spot. Many were English, but not all, and therefore it was hard to forge a common identity just out of that. There were also major regional differences that broke the colonies (and the early U.S.) into several parts, such as the North, South, and middle (later on adding a West.) People were also fiercely protective of their diverse religious beliefs, and tended to be loyal to the state the lived in rather than the colonies as a whole.
In short, the colonies that would later turn into the United States didn't have a strong "common ground" to fall back on other than a dissatisfaction with British rule, and even then not all colonists could agreed on that. To survive, it was necessary to create bonds that would bring these diverse interests together.
Attempts were made during the revolution to get the colonies to work together against the British, such as Franklin's "Join or Die." Still, when the war was over, the "founding fathers" created the Articles of Confederation and cemented the idea that the states were more important than the nation as a whole. Only when that failed, and the Federal Constitution was created, were people forced to see themselves as inextricably linked to one another.
Of course, the Civil War could be seen as having been caused by an imperfect nationalization process that left the North and South divided.
So, to sum it up, because America is so diverse it has sometimes been difficult for Americans to overcome their individuality in order to support the Nation as a whole.